Several methods for cooling and liquefying a natural gas stream to provide liquefied natural gas (LNG) are known. It is desirable to liquefy natural gas for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at high pressure.
In certain situations, particularly those where an off-shore natural gas field is located a significant distance from land, the cost of construction of an under-sea pipeline to a land-based liquefaction facility can render the extraction of natural gas from an off-shore field uneconomic. Floating Liquefied Natural Gas (FLNG) facilities have the potential to offer an economically viable solution to this problem by treating and liquefying the natural gas on an off-shore floating facility and supplying the LNG product directly to LNG tankers, thereby avoiding the costs involved in constructing a long under-sea pipeline. Large scale off-shore facilities for the liquefaction of natural gas may use direct-drive gas turbines to power the refrigerant compressors. Such gas turbines can pose a safety hazard, and placing the gas turbines adjacent to the LNG tanks should preferably be avoided.
“Safety drivers in the lay-out of floating LNG plants”, Third Topical Conference on Natural Gas Utilisation, AIChE Pub. No. 176, pp. 359-372 (2003) by Persaud et al, discloses that gas turbine blade and rotor failures are high energy events with the potential to penetrate the FLNG double deck and cause a fire in the cryogenic storage tanks in a worst-case scenario. For this reason, the authors recommend keeping the gas turbines away from cryogenic storage tanks.
However, when providing a FLNG facility, it is necessary to fit the liquefaction equipment into a much smaller footprint than would normally be available on-shore. For example, a FLNG facility may be restricted to about one-quarter of the size of an equivalent on-shore facility. It is therefore not a simple matter to locate the gas turbines sufficiently far away from the cryogenic storage tanks to satisfy safety considerations.
There is thus a drive in the industry to remove the need for using a gas turbine for driving a compressor.
WO 2006/052392 discloses a liquefaction facility on a liquefied natural gas transportation vessel, wherein natural gas is refrigerated by a refrigerant until it is condensed into a substantially liquid phase. In this process, the refrigerant is compressed in a compressor. Power is provided to the compressor by a power generator in the form of an engine. The engine may be gas powered, with the gas being provided from either naturally-occurring boil-off of natural gas from the LNG stored in the containment structure, or from an independent fuel supply. Alternatively, the engine may be diesel powered. The power generator drives a motor, which in turn provides mechanical power to the compressor.